From the U.S. Pat. No. 4,471,438 a hydrodynamic torque converter is known, the bridging clutch of which comprises a piston, which is guided axially displaceably on the turbine wheel hub and is sealed with respect to the hub. The piston forms, in the region of its external diameter opposite a drive-side casing wall of a converter casing, an engageable friction connection, which makes possible the direct transmission of torque from the casing to the turbine wheel by by-passing the hydrodynamic circuit. When the bridging clutch is disengaged the axial support of the piston is provided at a step of the turbine wheel hub. In this operating condition, the oil circuit of the converter is subjected to fluid pressure from the space between the piston and the drive-side casing wall, with the result that the piston is released and the converter fluid escapes via the converter chamber to a storage container. For engaging the bridging clutch, the direction of flow of the fluid is reversed. The fluid now supplied from the converter chamber produces a pressure difference relative to the piston chamber, which presses the piston axially against a friction surface on the drive side casing wall and the bridging clutch is closed. In particular, when the bridging clutch is opened, the casing of the converter is heavily loaded, because the axial force acting on the piston is transmitted directly to the casing wall remote from the drive side. Furthermore, the torque transmission capability of such a bridging clutch is limited by the value of the internal pressure, which on the one hand loads all the seals and on the other hand attempts to expand the entire casing.
It is the object of the present invention to improve a torque converter of the above named type of construction so that, on the one hand, either the transmission capability of the bridging clutch can be increased, without a pressure increase in the converter liquid being necessary, or, for the same transmission capability, the pressure can be reduced and, on the other hand, a lower loading for the casing can be achieved.